A. Field of Invention
The present invention relates, in general, to a system for testing electronic devices in which an electronic test head is positioned to connect to and dock with a handler of the device to be tested. More specifically, this invention relates to a pivot by which cable can be attached to and move with a movable test head.
B. Description of Related Art
In the automatic testing of integrated circuits and other electronic devices, special device handlers have been used which place the device to be tested in position. The electronic testing itself is done by a large and expensive automatic testing system which includes a test head which connects to and docks with the device handler. The test head uses high speed electronic timing signals so that the electronic circuits must be located as close as possible to the device under test. Accordingly, the test head is densely packaged with electronic circuits in order to achieve the high speed testing of the sophisticated devices.
A positioner, able to move along a support structure, carries the test head to the desired location at which the test head is positioned to connect to and dock with the device handler. The test head is attached to the positioner so that the test head can achieve six degrees of freedom.
A major problem is connecting a heavy cable, extending from a cabinet containing part of the automatic testing system, into the test head. Because the test head can be moved to a desired location and into a desired position, the heavy cable must move with the test head.
Conventional arrangements used to provide synchronized movement between the test head and cable fall generally into two categories: cable pivot-type and tumble-type arrangements.
In the tumble type, the cable is slung beneath the test head so that it connects to the side of the test head opposite the mechanism by which the test head is attached, at its center of gravity, to the positioner. This arrangement avoids the complexity and expense of a cable-pivot type approach. It allows the cable to hang beneath the test head to beyond the side of the test head, however, for a number of different orientations of the test head. Thus, the cable often gets in the way of the operator of the test system and very large test systems are limited in how low they may be positioned before the cables will touch the floor when the test head is in its down position. Moreover, the tumble-type arrangement requires a lengthy cable because it extends completely across the bottom of the test head for certain orientations of the test head.
A variety of cable pivot-type arrangements are known. In one, the test head is also held at its center of gravity by the positioner. The cable and positioner are attached to the test head on opposite ends of the test head. One disadvantage of this type of arrangement is that it cannot access, from underneath, horizontal plane handlers which are built like two pedestal office desks; the positioner stands where one pedestal of such a desk would be located.
A second cable pivot-type arrangement involves passing the cable through the mechanism by which the test head is attached to the positioner. This mechanism is arranged to permit pivotal movement of the test head. Typically, such mechanisms include an inner ring which, through bearings, is spaced from and able to rotate in an outer ring. One disadvantage of these mechanisms is that installation or removal of the test head requires that the cables be unplugged from the test system. The task of decabling and recabling a test system may take one half to one full day, puts a very expensive system out of commission, and risks damage. Another disadvantage is that the mechanism is expensive: the bearings themselves and the precise machining necessary to load the bearings in the ring are costly.
In order to avoid the necessity of unplugging the cables, it is known to provide the rings as split rings, connected by a C-clamp. When connected, the geometry of the split ring components makes it difficult to mount the ring directly to the test head; instead, additional structure is required to mount the test head to the inner diameter of the ring.
With the above discussion in mind, it is one object of the present invention to provide a device testing system which protects the cables which interconnect the test head to the test system. Continual bending, twisting, and insertion/removal of such cables is inherent when the system is used to test a variety of probers and handlers. Accordingly, the fatigue life of the cables is an important concern. Because fatigue life of materials from which cables are made decreases with increasing stress and the applied stress is proportional to the length of cable over which a bend or twist occurs, it is advantageous to control the length of the cable encompassing the bend or twist.
Another object is safety and ease of operation of the test head positioning system. Related objects are to allow installation or removal of the test head without unplugging the test head cable from the test system and to provide a system which properly balances the test head in the cable pivot arrangement for acceptable handling. Still another object is to provide a system able to test a variety of probers and handlers, including those built like pedestal desks, and to position especially large test heads low to the ground without having the cables touch the floor. It is also advantageous to allow mounting of the test head directly to the cable pivot arrangement. Finally, an object of the present invention is to provide a system which can be manufactured cost efficiently, especially by eliminating alignment tolerance requirements.